1. Field of the Invention
The present invention relates to shielded electric wires, and more particularly, to improvements in shielded electric wires, such as coaxial cables, etc., to be used in wiring electronic devices such as computers.
2. Description of the Prior Art
With the advent of large-sized computers and increased computer speeds, it has become desirable to increase the speed at which signals are transmitted over coaxial cables used in computer systems.
Transmission speeds, defined in terms of propagation delay time, should be 3.9 nsec/m or less and should preferably be 3.7 nsec/m or less. The propagation delay time is determined as a function of the dielectric constant of the insulating material for the coaxial cable and its thickness. When the insulating material is polyethylene or polypropylene, it must be expanded (by foaming) as high as 65% and 80%, respectively, to obtain the desired transmission speed. Such a high degree of expansion can be produced by extrusion expansion of polyethylene or polypropylene with a rigid skin layer provided on the outer periphery thereof.
In order to decrease the outer diameter of the coaxial cable, it is desirable to apply a closed shield by winding a number of fine electrically conductive wires on the outer periphery of the insulating layer. During the process of applying the closed shield, the insulating layer is pressed by the electrically conductive wires, resulting in a decrease in the degree of expansion of the insulating layer. This greatly increases the dielectric constant of the insulating layer, particularly with high expansion insulating layers.
Accordingly, even if a high expansion insulating layer is provided, the ultimate coaxial cable does not have the desired dielectric constant.